This invention relates to a method for the preparation of 3-chloro-4,5-difluorobenzonitrile. The compounds prepared in accordance with the invention are useful as intermediates in the preparation of various chemical products, in particular, in the preparation of substituted diphenyl ethers having herbicidal properties.
A prior art method for preparation of the compound, 3-chloro-4,5-difluorobenzotrifluoride and the use thereof as an intermediate in the manufacture of herbicidal diphenyl ethers is disclosed in U.S. Pat. No. 4,388,472. The preparation disclosed therein involves the steps of (1) reacting chlorine with p-trifluoromethylaniline in glacial acetic acid to form 2,6-dichloro-4-trifluoromethylaniline; (2) diazotization of the latter; (3) reaction of the diazo product with cuprous chloride in concentrated hydrochloric acid to form 3,4,5-trichlorobenzotrifluoride and (4) reaction with KF to form 3-chloro-4,5-difluorobenzotrifluoride. The patent further discloses the preparation of 3,5-dichloro-4-fluorobenzotrifluoride by diazotization of 2,6-dichloro-4-trifluoromethylaniline followed by reaction with sodium fluoroborate and decomposition of the product. Such methods, although useful for laboratory preparations are less suitable for commercial scale preparations. The starting material, p-trifluoromethylaniline, is expensive. Furthermore, the diazotization reaction is expensive and not readily adaptable to commercial scale. Furthermore, step 4 of the prior art route of synthesis, that is, the reaction of KF with 3,4,5-trichlorobenzotrifluoride, leads to the formation of an isometric mixture of 3-chloro-4,5-difluorobenzotrifluoride and 4-chloro-3,5-difluorobenzotrifluoride, and thus requires additional separation procedures. For similar considerations, the prior art synthetic route utilizing diazotization and halogenation in a manner similar to that detailed above for the preparation of 3-chloro-3,4-difluorobenzotrifluoride would also be unsatisfactory, if applied to the analogous commercial scale preparation of 3-chloro-4,5-difluorobenzonitrile.
Halogen exchange reactions wherein an alkali metal fluoride, such as KF, is reacted with a chloro-aromatic compound to form the corresponding fluoro-aromatic compound are well-known in the prior art. A typical example of such reactions is that described above wherein KF is reacted with 3,4,5-trichlorobenzotrifluoride to form 3-chloro-4,5-difluorobenzotrifluoride (U.S. Pat. No. 4,388,472).
Chlorodenitration reactions wherein a chlorinating agent is reacted with a nitro-aromatic compound to form the corresponding chloro-aromatic compound, are known in the prior art. See, for example, U.S. Pat. No. 4,470,930, which discloses the vapor phase chlorodenitration of nitro-benzonitriles to form the corresponding chloro-benzonitriles. The liquid phase chlorodenitration of 2-nitrobenzonitriles to form the corresponding 2-chlorobenzonitriles is disclosed in U.S. Pat. No. 4,225,534.
The compound prepared in accordance with this invention, that is 3-chloro-4,5-difluorobenzonitrile is a useful intermediate for the preparation of herbicidal diphenyl ethers, for example, of the type disclosed in European Patent Application Publication No. 0 23 392, and U.S. Pat. No. 4,388,472.
Accordingly, it is an object of this invention to provide a synthetic route for the preparation of high purity, substantially isomer-free 3-chloro-4,5-difluorobenzonitrile that is well suited for both laboratory and commercial scale preparations. It is a further object to provide a method for the preparation of novel benzonitrile compounds.